The development of platelet factor 4 (PF4)/heparin (H) antibodies initiates the morbidity of heparin-induced thrombocytopenia (HIT). This application seeks to elucidate the cellular basis of the PF4/H immune resposne using an optimized murine immunization model developed in our laboratory. Studies with this model have shown that PF4/H ultra large complexes (ULCs) potently induce PF4/H antibody (Ab) formation, directly activate DCs and induce proliferation of antigen-naive T-cells. Based on these observations, we hypothesize that PF4/H ULCs are not processed and presented as conventional antigens, but activate the immune system as SAGS. To test this hypothesis, we propose the following